Heretofore various types of aircraft altimeters have been provided for indicating the altitude at which an aircraft is operating. In the so-called barometric type of altimeter a pressure or altitude sensitive barometric or aneroid bellows is provided. The bellows is a hermetically sealed unit having a diaphragm that is deflected in and out as the ambient barometric pressure varies.
A suitable visual indicator such as one or more pointers are operatively interconnected with the diaphragm by means of a suitable gear train and/or linkage mechanism. As the aircraft ascends and descends, the barometric pressure varies as a function of altitude and the diaphragm is deflected in or out whereby the indicator is moved to indicate the altitude of the aircraft.
If the aircraft is subjected to air traffic control, it is essential the controller know the altitude of the aircraft. Historically, it has been customary for the ground controller to periodically request the pilot to read the aircraft altimeter and relay the reading by radio to the controller.
More recently it has been proposed certain aircraft have a so-called transponder capable of automatically transmitting certain types of information or data such as aircraft altitude to the ground based traffic control station in response to an interrogating signal. Heretofore, in order to accomplish this it has been necessary to add to the aircraft a special altitude encoder which functions separate and distinct from the existing altimeter. Since the altimeter and the encoder are separate units, they may or may not read the same. Alternatively it has been proposed to design a completely new altimeter and encoder. This necessitates designing and obtaining a completely new set of tools, production procedures, inventory of spare parts, servicing procedures, etc. As a result, it has been extremely expensive to produce an acceptable encoder.